This invention relates to vertical-cavity surface emitting lasers (VCSELs) and, more particularly, to a VCSEL wafer having both bottom-emitting and top-emitting VCSELs formed thereon, the top-emitting VCSELs being used for performance testing purposes.
It is desirable to integrate vertical-cavity surface emitting lasers (VCSELs) on semiconductor circuit chips (usually Si CMOS) via flip-chip bonding to reduce packaging costs, increase performance by eliminating parasitic circuit elements such as wire bonds, and allow for the formation of integrated transmitter arrays. It would be particularly useful to integrate 850 nm VCSELs since that wavelength forms a standard for local area fiber-optic networks (the other standard wavelength is 1.3 xcexcm, where no manufactured VCSEL technology yet exists). In a flip-chip bonded VCSEL, the light must be emitted through the bottom surface, i.e., the substrate side of the VCSEL chip. For 850 nm operation then, the opaque GaAs substrate must be removed. The GaAs substrate is removed after the VCSEL chip is flip-chip bonded to a semiconductor chip (see, e.g., U.S. Pat. No. 5,385,632, issued on Jan. 31, 1995). Unfortunately, this means that the VCSEL chips cannot be tested until they have already been mounted. This destroys one of the major advantages of VCSELs over edge-emitting lasers, which is that VCSELs can be tested on the wafer before cutting the wafer into individual chips. Moreover, if the VCSEL is bad then the bonded VCSEL/semiconductor chip must be discarded. What is desired is a technique to test VCSEL devices while they are still part of the wafer.
In accordance with the present invention, we have overcome the above-described problem using a technique which determines the performance of substrate-side (or bottom) emitting VCSELs while they are still part of the wafer. The technique involves forming top-emitting VCSELs on the same wafer as bottom-emitting VCSELs and then testing the top-emitting VCSELs and using the results to determine the performance of the bottom-emitting VCSELs of the wafer. The test results of the top-emitting VCSELs have been found to correlate well with the bottom-emitting VCSELs performance and to be a good indicator of the lasing wavelength and performance variations over the wafer. Using this technique once the test results of the top-emitting test VCSELs are determined, then the performance of the bottom-emitting VCSEL devices on the wafer can be determined and, if acceptable, the wafer can then be sawed or cut into individual VCSEL chips and utilized without individual testing of the VCSEL chips.
More particularly, in accordance with the present invention, a VCSEL wafer (and a method of forming the wafer) is disclosed including a plurality of substrate-side emitting VCSELs formed on the wafer, each including a top mirror having a metal reflecting cap formed thereon, the wafer also includes at least one VCSEL constructed having a top mirror without a metal reflecting cap formed thereon to enable top-surface emitting therefrom.
According to another feature of the invention, the non-capped VCSELs are formed in saw-cut regions of the wafer. Other features describe the formation of the top and bottom mirrors of the VCSEL using a variety of materials and specify reflectivities for the top and bottom mirrors.